The present invention relates to a ball screw shaft, provided with means for dissipating vibrations, of the kind having a longitudinal cylindrical internal cavity, closed at its ends by means of inserted terminals or plugs.
In ball screw devices of the kind with rotating nut screw, as well as those in which the shaft rotates, the natural vibration frequency of the shaft remains low. Therefore, when the frequency of small vibrations, applied to the shaft by the nut screw through its vibration, are in area of the critical velocity of the shaft, such resonance phenomena occur as to cause large vibrations in the screw shaft because of the great distance between the supports of the shaft.
Within the field of known solutions to the problem, it is found that the unexamined Japanese patent publication N. 6-11009 provides a screw shaft bearing, in one its longitudinal cavity, a long stem serving as vibration controlling mass member, which comes in contact with the inner surface of said cavity when the shaft screw vibrates. Said collision limits the vibration of the screw shaft in its resonance point, which shaft is therefore able to operate at velocities exceeding its critical velocity. However, when the screw shaft is excited at low frequency, the screw shaft and the vibration controlling mass member do not always move relative to each other so as to touch each other, and in the aforementioned patent no concrete means are indicated to allow the screw shaft and the vibration controlling mass element to move in such a way as to expect such a behaviour.
Subsequently, U.S. Pat. No. 5,613,400 has tackled the same problem of damping the transmission of vibrations from a nut screw to a screw shaft to prevent it from resonating and allow an operation with a long travel and high speed. This patent discloses a ball screw device having a screw shaft including a longitudinal internal cavity with a longitudinal vibration controlling mass member within it, held by support bushings, and a damping element interposed between the vibration controlling mass member and the inner surface of the longitudinal cavity of the screw shaft to prevent contact between the vibration controlling mass member and the longitudinal cavity. According to the aforementioned patent, the vibration controlling mass member is constituted by a single rigid steel hold ring fastened to the ends of the screw shaft.
Although said steel hold ring, as a vibration controlling means, is provided associated to various damping and supporting means in different embodiments, nonetheless it cannot provide a satisfactory damping effect in a low frequency area and limit vibrations in a wide range of frequencies in correspondence with the natural frequency of the screw shaft that varies with the position of the nut screw.
The present invention is therefore aimed at solving the problem stated above, overcoming the limits and drawbacks of the prior art.
Thus, the main aim of the invention is to provide dissipater means able to transform the vibration energy into other forms of energy, especially into thermal energy.
Another aim of the invention is to increase the strength of the screw shaft at equal weights thereof.
An additional aim of the invention is to reduce the weight of the shaft relative to other known hollow shafts of equal size, consequently to obtain a reduction of deflection, so that the shaft can have a higher frequency than said other shafts.
The aims are achieved by the present invention which provides a ball screw shaft with increased vibration frequency and improved vibration dissipation, of the kind having a longitudinal cylindrical internal cavity, closed by means of inserted terminals or plugs at the ends of the screw shaft and housing vibration controlling means, which, from a general point of view, is characterized in that said screw shaft has in said longitudinal cylindrical internal cavity a filling of granular material with low density and high superficial roughness freely distributed and, moreover, according to a variation of the invention, said longitudinal cylindrical has inserted therein a pipe made of composite polymeric material, whose outer surface is made adherent and integral with the inner wall of the cylindrical longitudinal cavity; said pipe made of composite material having in its longitudinal cavity a filling of granular material with low density and high superficial roughness freely distributed.